October 1998

BY JOSH ADELSON

Cost-savings are a major driver behind the adoption of IP telephony, and fax is a large part of telecommunications costs. Fax remains the preferred method for many document transfers, in spite of the rise of alternatives such as e-mail. According to industry estimates, businesses spend $30 billion annually on fax transmissions in the U.S. alone; fax accounts for approximately a third of all long-distance business calls and a higher percentage of international calls.

We're all familiar with the cost argument for IP-based fax by now. In addition to these economic incentives, IP fax presents an opportunity because it is less vulnerable to the networking issues that challenge IP voice. Specifically, where two-way voice is highly delay-intolerant, fax can operate across a span of network conditions including low, moderate and even high latency. The technologies available for IP fax provide a framework for determining which technology makes the most sense, based on network characteristics and user applications. Implementation options, such as outsourcing to a service provider, and emerging IP fax standards and technologies also play into deriving benefits from IP fax today.

APPROACHES TO IP FAX
There are three main approaches to transmitting fax over IP networks: real-time "demod/remod," real-time with spoofing, and store and forward. All three approaches are legitimate. What drives the selection are the delay characteristics of the network and the intended value-add of the system.

Real-Time Demod/Remod
Demod/Remod, also known as fax relay, is the simplest method. In this approach, the fax signal is simply demodulated for transmission over an IP network pipe that has been established by an IP telephony gateway, and then remodulated at the remote end for the receiving fax. Demod/remod is typically deployed in voice-centric systems where the network delay is assumed to be minimal, such as providing a fax capability within a voice gateway.

Real-Time With Spoofing
Spoofing is a technique by which IP fax gateway devices intelligently transmit signals to their respective local fax machines in order to maintain a connection that would otherwise time out due to delays in the network. Spoofing makes real-time applications possible in near-real-time conditions. Good spoofing techniques provide nearly 100 percent completion rates under delays as high as two seconds, which covers even most public Internet connections. Spoofing is also useful in low-delay environments because it provides greater assurance of successful transmissions in the event of unexpected network delay.

Even when spoofing is able to maintain the fax session, network delays will increase the total duration of the call, an effect known as elongation. The actual amount of elongation depends on a number of factors including system delay, the number of pages and the number of transitions between compression formats. On a typical text transmission with system delay of 400 ms (considered the boundary delay for successful voice over IP), elongation will add about two seconds to the first page, and about one second for each additional page. Under system delays of two seconds, the same transmission will have greater elongation - perhaps 10-12 seconds for the first page and four seconds for each subsequent page.

Elongation is generally a moderate inconvenience and is considered a tolerable side effect outweighed by the benefits that IP fax offers. It may become an issue for IP fax service providers who use time as a basis for billing. These providers must take elongation into account in order to establish competitive rates; alternatively, they may choose to charge by the number of pages sent rather than by call duration.

Store-and-Forward
With store-and-forward technology, fax becomes completely non-real-time, using instead an electronic mail-like system in which the image is compressed into a file and transported. Store-and-forward therefore works under any network delay condition. However, this method sacrifices some of the principle benefits of fax, namely:

• immediate delivery;
• immediate confirmation, by displaying or printing a confirmation message on the sender's fax machine; and
• feature negotiation; because store-and-forward de-couples the session between sending and receiving fax machines, feature negotiation is not possible, and machines must revert to the lowest common denominator page sizes and image formats.

However, feature negotiation in store-and-forward applications can be achieved if the gateway uses intelligent fax board, that is capable of on-the-fly format conversions.

In spite of these limitations, however, store-and-forward IP fax opens the door to new benefits, such as never busy, broadcast, and integration with e-mail client software.

IP FAX APPLICATIONS
Armed with an understanding of the basic technology options, the next step is to examine the current and planned use of fax in one's own environment. A simple but effective way to segment fax applications is by the sending and receiving devices: traditional facsimile machines vs. personal computers. In this model, there are just four possible scenarios of fax communication: fax machine to fax machine, fax machine to PC, PC to fax machine, and PC to PC. Each of these scenarios drives its own most appropriate IP fax approach.

Fax Machine To Fax Machine
This is the traditional scenario. The user's expectation is that the fax is received in real time. In fact, the real-time nature of fax might be the main motivation for using it in the first place. Therefore, fax machine to fax machine transmission favors a real-time approach. If the network is reliably low-latency, then a demod/remod solution might be suitable. If the network has variable latency, such as a corporate intranet that is also used for data, then spoofing must be employed.

Store-and-forward solutions might be attractive for the cost savings they offer, especially when deployed over the Internet. If a store-and-forward approach is taken between fax machines, alternate confirmation methods (for example web-based), must be implemented, and users need to be educated on how the system operates.

Fax Machine To PC
This scenario is difficult to treat, because the sender has no control or even knowledge of the destination device. The segment from the fax machine to the destination fax server may be real-time or store-and-forward. The segment from the fax server to the PC is by definition store-and-forward because fax server-to-client communication operates in a store-and-forward fashion.

Implementers probably only have control over one of the two segments. For the first segment, all the same considerations apply to the fax-to-fax scenario, favoring real-time but allowing store-and-forward. For the second segment, store-and-forward is really the only option.

PC To Fax Machine
When the originating device is a LAN-attached PC, the solution is always store-and-forward because it relies on networked fax servers that spool the images prior to transmitting them. Store-and-forward is viable in this environment, for several reasons:

• The originating user may not have the same real-time expectation, as the most familiar form of PC communication - e-mail - is store-and-forward.
• The PC user interface, unlike that of the fax machine, has a rich set of capabilities to inform the sender of the status of their fax.
• A PC user, having sent a fax, is likely to remain close to their PC, available to receive notification messages and act upon them. Fax machines, on the other hand, are centrally located and people generally walk away from them after sending the fax.

These are factors that make store-and-forward fax viable. Beyond these, store-and-forward holds genuine advantages in a PC-origination environment, compared to real-time approaches:

• Store-and-forward client software can be integrated into client software such as Microsoft Outlook or Lotus Notes.
• Like e-mail, store-and-forward fax is "never busy" because there is no need for the destination line to be available at the time of sending. If the destination is busy, the fax is simply stored in network fax servers and re-transmitted later. This is especially attractive to SOHO-based users who cannot afford to tie up scarce phone lines with multiple fax call attempts.
• Store-and-forward fax supports value-added features such as broadcasting and scheduling.
• Store-and-forward consumes idle bandwidth as opposed to extra bandwidth.
• Store-and-forward IP fax is similar to more traditional LAN fax - an option that will be discussed later in this article.

PC To PC
When both the sending and receiving ends are PCs, fax is rarely necessary because e-mail can be used.

OPTIONS FOR IMPLEMENTATION
A number of implementation options are available, based on the application and technical approach chosen. Real-time IP fax is generally implemented via IP fax gateways that sit between a PBX and the data network. Based on the outgoing fax destination phone number, the call is either routed traditionally to the PSTN or alternatively to the fax gateway, which demodulates and packetizes the fax transmission and sends it to a receiving gateway at the destination end.

A gateway might be dedicated to fax or it might be a voice over IP gateway/router that supports fax. Voice-centric products often implement a simple "demod/remod" fax capability, which assumes low latency in the network. Dedicated fax gateways are more likely to include advanced spoofing techniques to compensate for network latency.

Store-and-forward fax implementations rely on LAN-connected servers. These represent extensions to non-IP LAN fax servers, which have existed for several years. Considerations involved in selecting store-and-forward fax servers include operating system support, integration with popular client-server software and advance features such as broadcast and scheduled delivery.

Users who are evaluating store-and-forward solutions are also advised to examine traditional LAN fax products. These products are mature and offer a rich feature set. They can achieve cost savings by performing least-cost routing across the corporate Intranet. For example, a user at corporate headquarters in London may wish to fax a document to a customer in San Francisco. Through a distributed network of LAN fax servers, the document can be routed across the corporate network to the fax server nearest the destination-for example the corporation's sales office in San Jose. From here, the document is transmitted to the customer in San Francisco. The corporation only incurs PSTN charges for the 'last mile' between San Francisco and San Jose.

In general, the distinction between traditional LAN fax and IP store-and-forward fax will become increasingly blurred. Users will need to investigate viable offerings from both categories when they seek to implement a store-and-forward solution.

For both store-and-forward servers and real-time gateways it is important to consider the number of ports supported so that the appropriate scale solution can be implemented for each location on the network.

Service providers might be an attractive alternative to purchasing the technology. Many specialized IP fax providers have emerged to provide low-cost fax transmission over the Internet or over private IP networks. Using a service provider eliminates the need to select, manage and upgrade equipment, software and access lines. In addition, many service providers offer attractive service features, such as private fax mailboxes, home-based access through an ISP and calling cards.

Considerations involved in selecting a service provider include the reach of the service provider's network, delivery and uptime guarantees, and of course, the rates charged. For store-and-forward service providers (as well as for store-and-forward products), it is also important to evaluate delivery confirmation options. Service providers and product vendors offer a variety of methods including e-mail, web-based, paging and others.

EMERGING STANDARDS AND TECHNOLOGIES
The desire to send faxes over IP networks has not gone unnoticed by standards-setting bodies and technology vendors. Two major standards have emerged in IP fax: International Telecommunications Union (ITU) T.37 and T.38, addressing store-and-forward and real-time fax, respectively. T.37 uses Simple Mail Transfer Protocol (SMTP) addressing to send fax messages. As a result, T.37 provides a standardized means of integration into e-mail software packages. A T.37 "simple mode" offers no negotiation so only the lowest common denominator of image compression and page size formats are supported. In this mode, T.37 IP fax offers no confirmation of delivery. ITU also plans a T.37 "full mode" that will incorporate delivery confirmation and possibly some degree of feature negotiation.

The T.38 standard for real-time IP fax emerges from earlier standards that addressed real-time fax over X.25 packet data networks and from vendor-specific IP fax implementations. T.38 incorporates methods for ensuring successful call completion while minimizing call elongation.

On the technology front, facsimile vendors are starting to introduce fax machines that incorporate IP functions. For example, these devices have integral Ethernet ports and they allow the user to enter an e-mail destination. Besides reducing the need for gateways, these "Internet aware" fax machines eliminate certain delays that are created by the fax-to-gateway handoff. Another important technology trend for IP fax is the improvement of the Internet and private IP networks. For example, the Resource Reservation Protocol (RSVP) is a method by which network routers reserve bandwidth for specific traffic types, thereby providing quality of service in the network.

CONCLUSION
IP fax presents a great opportunity to reduce costs, improve communications, and gain experience that will be valuable in the deployment of voice over IP. To minimize the confusion, it's necessary to understand how fax is being used in your environment today, and what you want to achieve by moving to IP fax. The emergence of ITU standards, combined with technology vendor implementations is an indication that IP fax is here to stay and that a wide variety of interoperable solutions will be available to satisfy user needs.

Josh Adelson is the market development manager, IP Telephony, for Brooktrout Technology, Inc. Brooktrout is a leading supplier of software and hardware products for system vendors and service providers in the electronic messaging market. For more information, visit the company's web site at www.brooktrout.com or call 888-248-5401. E-mail comments to the author at jadelson@brooktrout.com.